1. Technical Field
The present invention relates to an optical system, an optical pickup device, and a recording and/or reproducing apparatus for a sound and/or image.
2. Technical Background
Conventionally, an optical disc known as a compact disc (CD) or a digital versatile disc (DVD) has been widely used for the recording of digital data, for example, accumulation of music information and visual information or the recording of computer data.
Therefore, because of the coming of information society, an increase of capacity of optical disc has been recently required strongly.
As to the optical disc, to improve a recording capacity (recording density) of data per unit area, a spot diameter of a converging spot obtained from an optical system for optical pickup is shortened. As well known, the spot diameter is proportional to λ/NA (here, λ denotes wavelength of light emitted from a light source, and NA denotes a numerical aperture of an objective lens). Therefore, to shorten the spot diameter, the shortening of wavelength of light emitted from a light source in the optical pickup device and the heightening of numerical aperture of an objective lens arranged oppositely to the optical disc are effective.
As to the shortening of wavelength in the light source, the research of a blue-violet semiconductor laser emitting a laser beam of wavelength of around 400 nm has been developed and will be put to practical use in near future.
However, in the optical system having the high numerical aperture or the shortened wavelength, an amount of aberration in the whole optical system is easily changed due to non-uniform or partially-changed thickness of a protective layer of the optical disc, a change in wavelength of light emitted from the light source, difference between light sources and/or a change in temperature of the optical system. Therefore, it is difficult to maintain a spot of light having small diameter.
In the optical pickup device, laser power in the recording of information is generally larger than that in the reproduction of information. Therefore, when a reproduction mode is changed to a recording mode, there is a case where mode hopping occurs. In this mode hopping, a central wavelength of light is immediately changed by several nanometers due to a change in output power. A change in focal position caused by the mode hopping can be obviated by focusing an objective lens. However, because it takes several nanoseconds for the focusing of the objective lens, a problem has arisen that the recording of information cannot be correctly performed due to the change in focal position in a period of time of several nanoseconds. The shorter the wavelength of light emitted from the light source is, the larger the change in focal position is. Therefore, the shorter the wavelength of light is, the larger the deterioration of the wave front aberration of the light due to the mode hopping is. Therefore, the optical pickup device using the blue-violet semiconductor laser as the light source requires the correction of the focal position of the converging spot which changes with the wavelength of light.
Also, when an objective lens having the high numerical aperture is made of a plastic lens, there is a probability that an amount of spherical aberration changes due to a change in temperature. In this case, the correction by moving a part of a converging optical system is possible. However, because the change in temperature occurs non-periodically, it is required to always observe the converging spot, the aberration and the temperature per se. As a result, cost of the manufacturing of the device is heightened.
To solve the problem of the change in wavelength, a ring-shaped diffraction pattern is formed on an optical surface of an objective lens, and axial chromatic aberration is corrected by using the diffraction function of the diffraction pattern. Therefore, the change in focal position caused by the change in temperature can be suppressed to a low degree in the optical pickup device using the blue-violet semiconductor laser as the light source. An objective lens of this optical pickup device is disclosed in a Published Unexamined Japanese Patent Application No. H09-311271.
Further, as an optical system relating to the correction for the change in temperature, an objective lens for correcting a change in wavelength of light of a light source caused by the change in temperature by using the diffraction is disclosed in a Published Unexamined Japanese Patent Application No. H11-337818.
However, because the diffraction pattern has a very fine structure, the fabrication of a mold for the objective lens having the diffraction pattern is difficult. Also, a problem has arisen that transmittance of light in the objective lens is easily lowered due to the effect of a fabrication error in the fabrication of the diffraction pattern.
Further, the finer the diffraction pattern is, the larger the effect of the fabrication error on the transmittance of light is. Therefore, because the diffraction pattern in a peripheral portion of the lens is finer than that in a central portion, the objective lens has a transmittance distribution having a low transmittance in the peripheral portion. As a result, a problem has arisen that the converging spot according to the transmittance distribution is enlarged.